A future mobile communications system that mainly carries out image or data communications requires capabilities far beyond the capability of the conventional mobile communications system (for example, an IMT-2000-based system). To this end, higher capacity, higher speed, broader band or the like have to be realized.
In a broadband mobile communications system, frequency selective fading caused by a multi-path transmission environment tends to be problematic, which makes an OFDM (Orthogonal Frequency Division Multiplexing) method be considered promising as a method of the next generation communications system. In the OFDM method, guard intervals are added to active symbols including information to be transmitted so as to produce symbols, which are in turn transmitted one by one at predetermined transmission time intervals (TTIs). Here, plural TTIs compose one frame. In addition, the guard interval is generated using part of information included in the active symbol. The guard interval may be called a cyclic prefix (CP) in some cases. FIG. 1 shows a relationship among the frame, the TTI, and the symbol. Since a receiver receives signals with various transmission delays, inter-symbol interference is caused. However, in the OFDM method, such inter-symbol interference can be sufficiently suppressed as long as the transmission delays fall within a time length of the guard interval.
During a time period of one TTI, various channels are transmitted. The channels may include a common pilot channel, a shared control channel, and a shared data channel. The common pilot channel is used by plural users to demodulate the shared control channel. Specifically, the common pilot channel is used for channel estimation, synchronous detection, reception signal quality measurement, or the like. The shared control channel is used to demodulate the shared data channel including payload (or traffic information channel). Regarding conventional signal formats including the pilot channel, see non-patent document 1, for example.    [Non-patent document 1] Keiji Tachikawa, “W-CDMA mobile communications method”, MARUZEN Co., Ltd., pp. 100-101.